The main purpose of this work is to study some of the factors involved in the regulation of the cellular levels of ribosomal components in mammalian cells. The model system is a series of hamster- mouse hybrid cells whose ratio of hamster/mouse chromosomes spans over a 50-fold range. The molecules being studied are the ribosomal RNA (rRNA) and ribosomal proteins (r-proteins) of cytoplasmic (cyt.) and mitochondrial (mit.) ribosomes. We have found a hybrid clone in which the ratio of radioactivity in hamster and mouse types of cyt. rRNA differs after short or long exposures to a radioactive precursor. We are currently trying to find out the mechanism of this phenomenon and why this does not occur in other hybrids. By two-dimensional gel electrophoresis we can detect a series of differences between hamster and mouse cyt. r-proteins. This method will be used to study the synthesis and content of species-specific cyt.r-proteins in these hybrids. We have found a hybrid clone in which the nuclear genome is predominantly mouse, synthesizing mostly mouse cyt.rRNA, and the mitochondrial genome is mostly hamster, synthesizing mainly hamster mit.rRNA. Two-dimensional gel electrophoresis will also be used to study species-specific mit.r-proteins in this hybrid and others. Hamster cells infected with adenovirus type 2 undergo a lytic interaction, while mouse cells do not. The reverse is true with polyoma virus. The hybrids which have been infected until now with the adenovirus type 2 or polyoma virus, while undergoing lysis, have not had any significant change in the hamster/mouse ratio of cyt.rRNA synthesis.